1. Field of the Invention
The present invention concerns a high pressure xenon ionization detector. More particularly, the present invention relates to the use of xenon at or near its critical point for detecting ionization.
2. Background Information
Heretofore the following detector systems were used to detect ionization for nuclear radiation: sodium iodide crystals, germanium crystals and cryogenic liquid noble gases.
Sodium iodide crystals were heretofore used to detect nuclear radiation, for example, gamma-rays. The efficiency of such detection system, however, was quite small, only a few percentage points, and the energy resolution was quite poor, i.e., 10.sup.5 electron volts for detection of 10.sup.6 electron volt gamma-rays. Large (quantitative) energy resolution results in poor discrimination of energy levels.
The use of intrinsic germanium crystal detectors proved to provide an acceptable energy resolution (approximately 1,600 electron volts), but the efficiency of germanium detection systems was very low, i.e. 10.sup.-3.
Cryogenic liquid noble gas ionization detectors have been built since the work of Norman Davidson and A. E. Larsh, Jr., Physical Review, 74, pp. 220, (1948) and Norman Davidson and A. E. Larsh, Jr., "Conductivity Pulses in Insulating Liquids by Ionizing Radiation", Physical Review, 77, 706-711, (1950). They used cryogenic liquid argon operating near the thermodynamic triple point. Later, L. S. Miller, S. Howe and W. E. Spear, "Charge Transport in Solid and Liquid Ar, Kr, and Xe", Physical Review, 166, 871-878, (1967), used cryogenic liquid xenon near its triple point to measure the ionization due to energetic electrons.
Triple Point of Ar : 83.8.degree. K., 517.15 mm Hg PA0 Critical Point of Ar : 150.7.degree. K., 48.6 bar PA0 Triple Point of Xe : 161.4.degree. K., 612.2 mm Hg PA0 Critical Point of Xe : 289.72.degree. K., 58.4 bar.
Several groups of physicists have built cryogenic liquid xenon or argon ionization detectors. All of these, however, have been designed to operate at or near the thermodynamic triple point of each liquid.
The technique of measurement of the ionization in a cryogenic liquid noble gas detector was developed by a group at University California at Irvine, Peter J. Doe, Hans-Jurg Mahler, Herbert H. Chen, "Observation of Tracks in a Two-Dimensional Liquid Argon Time Projection Chamber", Nuclear Instruments and Methods, 199, 639-642, (1982).
Cryogenic liquid xenon ionization detectors operating near the triple point (not the critical point) of xenon have been developed and studied by T. Takahashi, S. Konno, T, Hamada, M. Miyajima, S. Kubota, A. Nakamoto, A. Hitachi, E. Shibamura and T. Doke, "Average Energy Expanded Per Ion Pair in Liquid Xenon", Physical Review, 12, 1771-1775, (1975) and E. Shibamura, A. Hitachi, T. Doke, T. Takahashi, S. Kubota and M. Miyajima, "Drift Velocities of Electrons, Saturation Characteristics of Ionization and W-Values for Conversion Electrons in Liquid Argon, Liquid Argon-Gas Mixtures and Liquid Xenon", Nuclear Instruments and Methods, 131, 249-258, (1975).
The design characteristics of ionization detectors containing grids is described in O. Bunemann, T. E. Cranshaw and J. A. Harvey, "Design of Grid Ionization Chambers", Canadian Journal of Research, 27, 191-206, (1949).
Previous workers in the art have employed liquid xenon detector systems, but at low pressures, approximately one atmosphere, and at very low temperatures, near the triple point. The use of xenon at such low temperatures required extensive and expensive cooling system, vacuum cryostats and the need for precise temperature control. Also the energy resolution of such prior xenon systems is poor, i.e., 65,000 electron volts for detection of 1,000,000 electron volt gamma-rays.